This invention relates to an improvement in phase zone plate optics embracing contact and intraocular lenses. A "phase zone plate", as employed herein and in the claims, is a unitary optical region of a lens utilizing the combination of a zone plate and optical facets (such as in the form of echelettes) in the zones of the zone plate, and the combined facets in the zones diffract light to produce a specific wavefront which results in a specific intensity distribution of light at a variety of orders (e.g., 0.sup.th, 1st, etc.) of the zone plate. The orders constitute the foci of the zone plate. In a restrictive sense and also in the most utilitarian sense, the phase zone plate is designed for general lens applications where the distribution of light at effective intensities is dependent upon zone spacing for yellow light. Yellow light, as employed herein, is that portion of the visible spectrum at 530-570 manometers.
This invention relates inter alias to contact lenses. Contact lenses are classical vegence type lenses. They possess a concave corneal bowl (the posterior surface) that allows fitting to the eye and the outer surface (the anterior surface) is smooth and shaped to allow the eyelid to slide over the eye and to provide proper vergence of light (taking the lens material's refractive index into consideration) to a focal point accommodating to the eye. The majority of the commercial contact lenses are shaped such that the lenses are thinnest about the optical axis and the depth of the lenses gradually increases along a sloped radial length extending in the direction of the lens perimeter. Owing to the difference in depth extending from the optical axis, light passing through the optical axis has to pass through less of the lens material. Because light travels faster in air, the light passing through greater depths relative to light passing through lesser depths will be shifted, hence be retarded in time..sup.1 Consequently, the shape of the lens is selected to accommodate this progressive retardation of the light so that the lightwaves emanating from the posterior surface are in synchronization in reaching a desired focal point. FNT 1. See Fincham, et al., Optics, Published by Butterworths, London, 9.sup.th edition, 1980, 1981, pages 72-75.
This invention concerns contact lenses utilizing phase zone plate optics, such as phase zone plate bifocal and "tuned" Fresnel lenses making use of concentric annular zones. Such lenses generally follow the designs described, for example, by Allen L. Cohen in U.S. Pat. Nos. 4,210,391; 4,338,005; and 4,340,283 ("Cohen patents"). A Cohen lens design provides that the radii "r.sub.n " of the annular and concentric zones are substantially proportional to .sqroot.n and that the zones are cut so as to direct light to more than one focal point.
The Cohen lens design with phase zone plate optics allows bifocal lens constructions which are exceptionally thin. Contact lenses may be designed with phase zone plate optics in order to achieve a bifocal or other multifocal effects. The specific chromatic properties of a phase zone plate may be incorporated in the design of a contact lens including a contact lens having mutlifocal properties. All phase zone plate optical elements which are designated bifocals are possessed inherently with the ability to focus light to more than two focal points. They are designated bifocals because the intensity levels of the light to any two orders, e.g., the 0.sup.th and 1.sup.st order focal points are adequate for bifocal applications. In that sense, every bifocal distributes light to a third, and possibly more, focus. The judgment of whether a lens is a bifocal or trifocal is not based on any strict rule. If the wearer of the lens does not find objectionable the presence of the third or more focuses, then the lens is probably adequate as a bifocal..sup.2 FNT 2. See Klein and Ho, SPIE, August 1986, Table 2 and the comments about Table 2.
Other references mentioning or suggesting phase zone plate optics in regards to contact lenses are G. Forst, "Research into the Usability of Circular Grids as Aid to Vision," Der Augenoptiker, 1966 (12), page 9-19; Ziegler, "Fabrication or Correction of Optical Lenses," as modified by Cohen, see column 4, lines 27-36 of Cohen, U.S. Pat. No. 4,339,005, and column 5, line 63 to column 6, line 68, of Cohen, U.S. Pat. No. 4,210,391; Freeman, U.S. Pat. No. 4,637,697; and Freeman, U.S. Pat. No. 4,642,112 (to the extent that holography embraces phase zone plate optics).
Bifocal contact lenses utilizing the above principles of phase zone plate optics are commerically available. Such lenses are believed to utilize stepped annular facets each comprising a full-period zone where each zone has a depth of an optical path length of .lambda./2 providing a physical depth of .lambda./2(.eta.'-.eta.). .eta.' and .eta. are the indices of refraction of the lens and the medium (e.g., lachrymal layer) in which the lens is interacting and .lambda. is the design wavelength, in this case that of yellow light. This results in a bifocal contact lens where the 0.sup.th and 1.sup.st orders have an equal split of yellow light intensity at about 40.1%.
A full-period zone, for purposes of this invention, is defined as the smallest repetitive sequence of facets within a phase zone plate which are spaced substantially proportional to .sqroot.n. Such spacing is characterized by the formula: ##EQU1## where d represents the 1.sup.st order focal length. A half-period zone, for the purposes of this invention, is characterized by the formula: ##EQU2## where d represents the 1.sup.st order focal length.
Though the non-refractive step wall or riser to the plateau of the step is cylindrical or nearly cylindrical in the planar direction of the optical axis of the lens, and thereby occupies a small fraction of the lens phase zone plate surface area, it is regarded to be sufficiently large to contribute to a number of problems. Image shadowing and debris trapping are some of the problems that could be made less acute by reducing depth of the step wall or riser.
However, if such a lens were altered to reduce the depth of the facet.sup.3 to a value less than .lambda./2, .lambda. being the design wavelength, the optical qualities of the lens quickly becomes very poor. Though one is dealing with very small values when working at .lambda./2, a few millimicrons change in dimension seriously impacts on lens performance. For example, a 10% reduction in the depth of the depth of the facets yields a significant loss of effective bifocality in such a lens. It should be appreciated that all other values relating to the phase zone plate's dimensions are commensurately small. The plateau of the facet extending from the non-refractive step to the nadir of an adjacent non-refractive step, loses thickness from the .lambda./2 depth to a zero depth at the nadir of the adjacent step. These facts suggest that there is little one can do to avoid the loss in bifocality when reducing the depth of the step below .lambda./2. FNT 3. The terms "depth of a facet," "depth of the steps of the facet," "depth of the step" and words to that effect, as used herein, means in relation to the design wavelength of the lens, the degree of discontinuity in optical path length generated by the step.
It has been discovered that small changes (in some embodiments exceptionally small changes) in the configuration of the shape of inclination of the plateaus of the facets within the framework of .sqroot.n spacing provides that one can generate an effective bifocal lens based on phase zone plate optics where the depth of the step is less than .lambda./2. Through the alternating inclinations of facets embodied in the Cohen patents and the Cohen lens design, one can provide excellent bifocality in a contact lens where the facets have a depth less than about .lambda./2.
There is characterized herein a novel bifocal lens construction which provides the advantageous of r.sub.n zone spacing and stepped facets that introduce a discontinuity in optical path length of less than .lambda./2.
There is characterized herein a novel bifocal lens construction which reduces image shadowing attendant with conventional bifocal lenses containing stepped facets having a depth greater or equal to .lambda./2.
There is characterized herein a novel bifocal contact lens having a phase zone plate which possess less volume for tear collection than a bifocal contact lens having a conventional .lambda./2 parabolic echelette configuration. This means that the contact lenses of the invention have less volume for the trapping of debris on the eye and between the eye and the lens.
There are lens constructions according to this invention which provide surface contact with the cornea in a manner such that facet curves of the phase zone plate tangentially touch the cornea's surface. This causes the novel lens of the invention to rest more comfortably on the eye.
There is described herein a bifocal contact lens utilizing phase zone plate optics and a facet depth less than one-half the wavelength of the designed wavelength, where the primary focal points are at two orders, such as the 0.sup.th and 1.sup.st orders, the 0.sup.th and 2.sup.nd orders, or any other combination of two orders.